It's my best guess that many business card designers are still working within the technical restraints that existed in the color printing industry in the past. Solid colors are safe, and a smooth gradient fade is now technically possible.
But color printing technology has come much further than that. Intricate and subtle color printing is possible now. A bazillion interesting printed backgrounds are an easy and inexpensive way to jazz up even the most basic layout. Business cards can look better.
Maybe designers need less vector design and more raster design, and it's an Adobe Illustrator vs. Photoshop thing.